mechanism for adjusting the pre-load of a stiffening spring for seats

ABSTRACT

Mechanism for regulating the preload of at least a stiffening spring ( 3 ) for a revolving element ( 2 ) with respect to at least a support frame ( 1 ) of a seating device, of the type comprising a mobile striker ( 7 ) for one end of said at least one spring ( 3 ), connected to said revolving element ( 2 ), and a regulating cursor ( 6, 10 ), onto which the other end of said at least one spring ( 3 ) rests, said regulating cursor ( 6, 10 ) being conceived to slide along a rod-shaped guide element ( 4 ), characterised in that said rod-shaped guide element ( 4 ) is connected, at one end thereof, in a sliding manner, to said mobile striker ( 7 ), and it is also integrally fixed to said support frame ( 1 ).

FIELD OF THE INVENTION

The present invention relates to a mechanism for regulating (adjusting) the preload of a stiffening spring of a mobile element on a relative support frame belonging to a seating device, such as an office chair, for example.

It should be noted that hereafter the term “spring” refers to any type of non permanently deformable element, at, least in a direction defined by its two ends, for example when it is subject to traction or compression, presenting an elastic reaction by returning to its non-deformed conformation, at least along said direction. In this sense, said term can refer indifferently to cylindrical helical springs, flat spring or any other type of spring known to technical art, possessing the characteristic described above.

Furthermore, although in a general manner reference is made to elements of a seating device that is mobile with respect to a support frame, it should be noted that the term support frame does not necessarily refer to a fixed frame of the seating device, but simply a frame with respect to which the mobile element can perform a relative motion. Therefore, according to this definition, the support frame can be the frame that forms the base of the chair and the mobile element can alternatively be the backrest frame or the seating frame, or some other frame. Otherwise, the aforesaid support frame can coincide with a seating frame forming part of a synchronizing mechanism for chairs, wherein the mobile element is the backrest frame, even in cases where both the aforesaid backrest and seating frames can be rotated—precisely in a synchronised manner—with respect to the base of the chair.

PRIOR TECHNICAL ART

Prior art describes chairs equipped with a backrest that can be reclined with respect to the seat, for example, or having an arm rest that can pivot with respect to the chair base, or some other similar action, wherein a stiffening spring, with a pre-load that can be calibrated, is positioned between the seating frame, or any other support frame of the chair, and the relative backrest frame, or any other mobile element of said chair, aimed at controlling the slope of the aforesaid mobile element with a force that can be regulated by the user.

For example, the international patent application WO 02/032261 in the name of RODER, describes an office chair equipped with a synchronizing mechanism of the slope of the seating frame with the slope of the backrest frame, wherein a variable pre-load stiffening spring is provided to control the slope of the seating frame with respect to the base of the chair. The calibration of the preload of said stiffening spring in the RODER patent right, constituted in particular by a cylindrical helical spring, occurs thanks to the approaching or retreating of a regulating cursor for one end of the spring, to a striker for the other end of the spring, said striker being integral to the mobile element. The regulating cursor, constituted by a retainer element integral to one end of a rod that is in turn mounted telescopically inside a sleeve, is caused to translate with respect to a fixed striker integral to the aforesaid sleeve, thanks to a complex system of levers and cranks which can be operated by the user, using a control knob. The regulating cursor rod is also mobile with respect to the sleeve, or vice versa, during the rotation of the fixed element with respect to the mobile element.

Therefore, the mechanism described in the RODER application that provides that the possible stroke of the rod with respect to the sleeve, during the rotation of the mobile element, is modified because of the calibrating operations of the preload of the stiffening spring. This implies that in order to prevent the mobile element from rotating at different angles according to the calibration of the pre-selected spring, the rod stroke inside the sleeve, and therefore the dimensions of the rod and the sleeve, must be large enough to prevent the rod from reaching the end of stroke inside the sleeve, with resulting increase of the overall dimensions.

Furthermore, since said rod of the regulating cursor is subject to both the loads from the levers and cranks system for regulating the preload of the stiffening spring, as well as the force developed by the user during the rotation of the aforesaid mobile element of the chair, said rod must be appropriately dimensioned and suitably positioned within the mechanism of the chair, to prevent undue loss of the calibration required by the user, during the rotation of the mobile element of the chair.

Lastly, the preload regulating system of the spring resulted as not being easy to produce, given the complexity of the levers system involved. The regulating system of the patent application WO 02/032261 also involves frequent moving of the control knob with respect to the chair base, according to the calibration required by the user, with resulting difficulty of use for the user, who must search for and find the knob in a range of different positions when the pre-load needs to be adjusted.

The European patent application EP-A-1477028, by the Applicant, teaches, among others, how it is possible to realise a calibration mechanism of thee pre-load of a stiffening spring for a reclining element capable to be sloped with respect to a support frame of a chair, comprising a regulating cursor constituted by a threaded rod that is engaged, at one end, inside a nut-screw integral to the reclining element, and that has, at the other end, a retainer for one end of the stiffening spring. The spring is positioned between the regulating cursor and a striker realized on a support frame.

The rotation of the threaded rod, that can be for example controlled by a worm screw orthogonally engaging a toothed element integral to the same threaded rod, causes the translation of this latter with respect to the aforesaid nut-screw and therefore the regulation of the distance between the retainer of the regulating cursor and the striker of the support frame.

Said solution by DONATI, even if it is compact and effective, causes the translation of the rod of the regulating cursor, integral to the mobile element of the chair, within the kinematic mechanism of the chair itself, with the resulting need to take into consideration the dimensions and space required for such a translation.

Furthermore, since the rod of the regulating cursor is guided by the single nut-screw alone, it could become accidentally put out of alignment with respect to the nut-screw, this causing temporary jamming of the mechanism.

The USA patent application no. US 2004/0140703 to BOCK, describes a synchronizing mechanism for office chairs wherein a plurality of helical cylindrical springs is positioned between the seating frame and the backrest frame and wherein a preload regulation mechanism exists for said springs, comprising for each spring, a control cam integral in rotation to the seating frame and acting on a regulating rod for the spring. Said rod, integral in a sliding manner to a fixed striker for one end of the relative spring, is mounted with a mobile striker for the other end of the spring which, thanks to the cam action, translates to approach or retreat from the aforesaid fixed striker, in a manner to determine greater or lesser pre-loading of said spring. Thanks to the engaging system integral to the striker fixed to the backrest frame, the spring, thus loaded, provides an elastic counter action for the sloping of the backrest with respect to the seat.

This BOCK mechanism allows for the operation of a knob to rotate around a fixed axis to control the calibration of the springs in contrast with the slope of the backrest, but the relative sliding regulating rods slide freely, unprotected, inside the mechanism, with resulting possible problems of undesirable mechanical interference with the present members.

Furthermore, the sliding action of the control rods with respect to a guide that involves only a limited portion of the rod, can cause their undesirable movement in non-axial directions.

Lastly, the use of cams to regulate the sliding action of said regulating rods in the BOCK mechanism can cause inaccurate regulating of the calibration together with its relative instability.

The European patent application EP-A-0360763, to COFEMO, concerns a regulating mechanism of the pre-load calibration for a stiffening spring of the reclinable backrest of an office chair, that comprises a regulating cursor, having a resting surface for one end of the spring, which engages by means of threaded means with a guide rod, hinged to a fixed support of the backrest of the chair. The other end of the stiffening spring abuts against a mobile striker which is integral to the aforesaid backrest by means of a protection sleeve, telescopically engaged with an operating knob of the regulating cursor. The rotation of the regulating cursor, that is operated by the aforesaid operating knob, co-axial with the guide rod, determines the axial movement of said regulating cursor along the aforesaid guide rod and therefore the axial variation, under compression, of the length of the spring, thus causing the pre-load calibration, of the spring.

The COFEMO solution, does not provide for the movement of the guide rod inside the regulating mechanism of the chair, but the connection between the said guide rod and the fixed frame implies that the hinge used for said rod has mechanical characteristics such that they are able to withstand the forces, sometimes even considerable, exercised axially on the said guide rod, with the resulting over-size of the parts involved. However, any possible over-size does not prevent the fact that the said hinge between the guide rod and the fixed frame can become subject to wear.

Furthermore, the possible rotation of the guide rod, and therefore of the operating knob of the regulating cursor depending on the position assumed by the backrest, as well as the axial movement of said knob on the guide rod, caused by the operation of calibration of the stiffening spring, involves certain difficulties when operating the COFEMO mechanism for the user, who, depending on the load exercised on the backrest and the performed calibration, finds the operating knob in a different position each time he needs to adjust the calibration.

SUMMARY OF THE INVENTION

Therefore one aim of the present invention is to provide a mechanism for the preload regulation of a stiffening spring of a mobile element of a chair, with respect to a relative support frame, which does not present the drawbacks of prior technical art described above, and that is able to provide high precision and operating ease and that, at the same time, is structurally uncomplicated and that prevents the unguided sliding action of mobile elements inside the mechanism.

Another aim of the present invention is to provide a mechanism for the pre-load calibration of a stiffening spring of a mobile element of a chair, such as the backrest frame for example, which is particularly compact and easy to operate, for instance in a synchronizing mechanism of the backrest frame with the seating frame of an office chair.

These and other aims are achieved with a mechanism for the preload regulation of at least one stiffening spring according to the first independent claim and the successive subordinate claims.

The mechanism for regulating (adjusting) the preload of at least one stiffening spring for a element rotating with respect to at least one support frame of a seating device, according to the present invention, comprises:

-   -   a) a mobile striker for one end of the aforesaid stiffening         spring, which is constrained, preferably hinged, to the rotating         element of the seating device;     -   b) a regulating cursor onto which the other end of the         stiffening spring rests, and which slides on a rod-shaped guide         element.

Advantageously, this rod-shaped guide element is integrally fixed to the support frame and it is also constrained, in correspondence to one of its ends, at a distance from the integral link to the support frame, in a sliding manner, preferably in a telescopic manner, to a mobile striker.

In this manner, the rod-shaped guide element is fixed to both the support frame, to which it is linked in an integral mariner (and therefore in a manner similar to a fixed joint) preferably positioned at, or near, its end, as well as to the mobile element of the seating device, thanks to the fact that the mobile striker for the stiffening spring, constrained to said mobile element, is able to move with respect to the aforesaid rod-shaped guide element. This configuration, which can be implemented in a geometrically compact and structurally simple manner, provides the user with easy operation of the regulating cursor of the stiffening spring.

In particular, in the preferred example of telescopic coupling of the rod-shaped guide element with the aforesaid mobile striker, the compact nature of the structure thus obtained, is combined with the extreme precision of the regulating mechanism, that results substantially free of any possible jamming, given the substantial impossibility of out-of-alignment between the rod-shaped guide element and the mobile striker.

It should be noted that the term “telescopic” refers herein and hereafter to a system constituted of at least two sliding elements with respect to one another, wherein one of the elements is mounted at least partially inside the other, in such a manner to exit or return inside the latter during the relative sliding action of said elements. For example, a system constituted of a rod connected in a sliding manner inside a sleeve is covered by the concept of “telescopic system” used herein.

According to a preferred aspect of the present invention, furthermore, the mechanism foresees a manual operating knob kinematically connected to a control shaft and fixed in rotation to the aforesaid support frame, as well as means to transform the rotary motion of the aforesaid control shaft around its own axis into a movement that is—at least—a translation movement of the regulating cursor along the rod-shaped guide element.

In a preferred embodiment of the present invention, said means used to transform the rotary motion into a translation movement can comprise a mobile head equipped externally with grooves having a substantially helical form that couple with the teeth of a worm screw, obtained on at least part of the aforesaid control shaft. The mobile head with the helical grooves is connected in a sliding manner to the rod-shaped guide element and therefore also forms part of the aforesaid regulating cursor for the stiffening spring.

The use of these means to transform the rotary motion of the control rod linked to the operating knob into the translation movement of the regulating cursor along the rod-shaped guide element, is able to provide extreme precision in the preload regulation of the stiffening spring.

According to another aspect, of the present invention, the stiffening spring is a helical cylindrical spring, mounted in a manner to be co-axial with the telescopic system formed by the aforesaid regulating cursor and by the complementary fixed element.

In a preferred embodiment of the present invention, the aforesaid mobile element is the backrest frame of a synchronizing mechanism and the support frame is the seating frame of an office chair.

BRIEF DESCRIPTION OF THE FIGURES

A preferred embodiment of the present invention will now be described by way of a non-limiting example with reference to the appended drawings, wherein:

FIG. 1 is a cutaway view, seen from above, of an apparatus for an office chair comprising a mechanism for regulating the preload of a stiffening spring according to a particular aspect of the present invention;

FIG. 2 is a partial cutaway side view of the mechanism shown in FIG. 1; and

FIG. 3 is a schematic side view in cross-section of the mechanism shown in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

With general reference to the appended figures, according to a particular aspect of the present invention, the mechanism for regulating (adjusting) the preload of at least one stiffening spring 3 for a mobile element 2 of a chair, such as a backrest frame for example, with respect to a support frame 1, such as a seating frame for example, illustrated herein, comprises a mobile striker 7 which is connected to a mobile element 2 and onto which one end of a spring 3 rests, or in any case it is linked to, and a resting element (or linking element) 6, mounted in a sliding manner on a rod-shaped guide element 4 constrained to the support frame 1, for the other end of the spring 3.

The spring 3 is of the type wherein the distance between the two ends establishes the pre-load, in other words said distance determines the intensity of the elastic reaction force exercised by the said spring 3 when it is compressed further or placed in traction. Therefore, the variation of the distance between the mobile striker 7 and resting element 6, permits the regulation of said pre-load, making the said spring 3 more or less stiffen with respect to the mobile element 2, as will be shown further on.

In the embodiment described herein, said spring 3, is in particular, a helical cylindrical torsion spring that is mounted in a coaxial manner on the rod-shaped guide element 4, between the mobile striker 7 and resting element 6. Naturally, any other arrangement of the spring 3 between the two strikers 6 and 7 can be realised as an alternative, on condition that the variation in the distance between the two strikers 6 and 7 establishes the variation in the length of the spring 3, in its pre-loaded configuration.

The resting element 6, together with means for its motion, at least in translation along the guide element 4, as will be illustrated, therefore forms part of a regulating cursor 6, 10 which, by sliding along the guide element 4, establishes the variation in the distance between the two ends of the spring 3.

It should be noted that although herein we have described a mechanism for the preload regulation of a single stiffening spring 3, said mechanism can be easily adapted by those skilled in the art, to regulate several stiffening springs 3 with no difficulty.

As can be seen in the figures, in the particular embodiment illustrated herein, the mobile striker 7 is in particular constrained, by means of a pin 14, in an orthogonal position to the rod-shaped guide element 4, to the backrest frame 2, or to a part thereof, while the rod-shaped guide element 4 of the regulating cursor 6, 10, constituted of a preferably rigid rod, is advantageously fixed in a manner substantially integral to the seating frame 1, and at the same time is engaged, by means of a sliding link (i.e. a sliding block), with the mobile striker 7.

More in particular, the rod 4 of the present embodiment is constrained to the seating frame 1 thanks to the engagement of the end of a pin 17 inside a relative housing 16 set on the seating frame 1, and it is also constrained in a sliding manner to the mobile striker 7 which, as can be seen for example in FIG. 3, is constituted of an annular projection from a sleeve 5. The rod 4, at the end opposite to the end integral to the seating frame 1, is linked in a telescopic manner in the hole 15 of the sleeve 5. The kinematic mechanism formed in this manner therefore involves the rod 4 being fixed with respect to sleeve 5, which, during the reclining (tilting) action of the backrest frame 2, moves along said guide rod 4, thanks to the rod being engaged in the relative hole 15. To prevent possible and undesirable rotation of the guide rod 4 inside the hole 15 of the sleeve 5, both the rod 4, qt its end which is engaged inside the sleeve 5, as well as the hole 15 can assume a quadrangular section.

As any person skilled in the art is able to understand, even though the embodiment illustrated herein shows a guide element in the form of a rod 4 inserted in a sliding manner inside the hole 15 of a sleeve 5 integral to the striker 7 and constrained in rotation to the mobile element 2, it is possible—without any functional variation, of course—to realise a fixed rod-shaped guide element, having a co-axial hole inside which, a rod linked to a mobile element 2 and integral to the striker 7, can slide in a telescopic manner.

The regulating cursor 6, 10, constrained to slide along the rod 4, includes a resting element 6 for one end of the spring 3, constituted of a perforated disk inserted into the rod 4, which is moved by a mobile head 10, which is linked, at least in translation, along the rod 4, and is actuated by means of the kinematic mechanism 9, 10, 11, by a control knob 8. Said control knob 8 is in turn mounted on the seating frame 1, which is constrained to a relative base 12 of an office chair.

In the particular embodiment illustrated herein, the resting disk 6 is free to slide along the guide rod 4 and the mobile head 10 is arranged in contact with said resting disk 6, at the part opposite the part in which the spring 3 is engaged, and the mobile head 10 can be linked to the rod 4 by means of a threaded type coupling. For example, the mobile head 10 can be provided with an internal threading which is coupled with a complementary threading provided at least on the terminal part of the rod 4, in proximity to the constraint 16, 17, in such a manner that said mobile head 10 is able to roto-translate along said rod 4 in a manner controlled by said threaded coupling. Naturally, any other type of coupling between the mobile head 10 and the rod 4 that is able to guarantee the controlled translation action of the head 10 on the rod 4, remains within the required scope of protection for the present invention.

Therefore, although the mobile striker allows the transmission of the force exercised by the spring 3 on the backrest frame 2, the resting disk 6 transmits the force of the spring 3 onto the mobile head 10, which in turn transmits the force to the knob 8, which is constrained to the seating frame 1.

The rotation of the backrest frame 2 with respect to the seating frame 1, to which it is hinged, or with respect to the base 12, is thus counteracted by the spring 3, that opposes to any elongation or compression from its initial position established by the distance between the mobile striker 7 and the regulating cursor 6, 10.

In particular, as can be seen in FIGS. 2 and 3, in the embodiment illustrated herein, the spring 3 acts by elastically contrasting its compression, and the relative rotation of the backrest frame 2 with respect to the seating frame 1, or to the base 12, thus permitting the movement of the mobile striker 7 with respect to the regulating cursor 6, 10, with the resulting compression of the spring 3, which therefore progressively stiffens the backrest frame 2, thwarting the rotation of this latter with respect to the seating frame 1.

It is also possible to see that even though an indirect link of the mobile striker 6 with the seating frame 1 is described herein, any other type of constraint, whether direct or indirect, of said mobile striker 6 with any other element (support frame) of the chair, not integral to the mobile element 2 of the chair in question, remains within the scope of protection requested in the following claims.

Furthermore, even though the regulating cursor herein illustrated provides a resting disk 6 for the end of the spring 3 which is separated from the mobile head 10, any other type of configuration for said regulating cursor, such as its being realised in a single piece for example, remains within the protective scope of the present invention. The force (i.e. pre-load) with which, preferably but not necessarily, in a situation of no load on said backrest, the spring 3 counters the rotation of the frame 2 with respect to the frame 1, as stated previously, can be regulated by means of the variation of the distance of the mobile striker 7 from the regulating cursor 6, 10.

In the embodiment shown in the appended figures, the regulation of the distance between the resting disk 6 and the mobile striker 7 is obtained thanks to the controlled sliding of the mobile head 10 on the guide rod 4, provided by the engagement of a control rod 11, kinematically connected in rotation to the knob 8, with the aforesaid mobile head 10.

More in particular, the mobile head 10, co-axial with the guide rod 4, and incidental with said control rod 11, is induced to translate (or roto-translate, in the case where said head 10 is constrained to the rod 4 by a threaded type coupling), by gear means 10, 11, to transform the rotational movement of the control rod 11 into a movement of the head 10 that is at least translational, and therefore also of the resting disk 6, along the fixed rod 4, with respect to sleeve 5, and therefore also to the mobile striker 7.

In particular, said gear means can advantageously comprise, in the embodiment illustrated herein, a worm screw 11, made on at least part of the aforesaid control rod, which meshes in a substantially orthogonal manner, with the mobile head 10, appropriately equipped on its external surface with helical grooves and therefore, relative teeth.

The rotation of the worm screw 11 around its own axis, or in other words around the axis of the control rod, thus establishes the rotation and the translation of the head 10 along the fixed guide rod 4 with respect to sleeve 5. This roto-translation of the mobile head 10 is preferably controlled thanks to the threaded type coupling described above, between said head 10 and at least pad of the rod 4. The rotation direction of the worm screw 11 establishes the corresponding rotation direction of the head having the helical grooves 10 and therefore, the relative translation direction of the resting disk 6. Therefore, given the limiting connection between the worm screw control rod 11 and helical groove head 10, and precisely of the gear type, the rotation in one direction or another of the control rod 11, imposed by the manual actuation device 8, causes the travel in one direction or the other direction of the mobile head 10, with the resulting approach or retreat of the resting disk 6, maintained in engaged position with the head 10 by the spring 3, with respect to the mobile striker 7.

The worm screw 11, which can be seen in particular in FIG. 2, is actuated by a handle 8, connected in a revolving manner, as stated previously, to the seating frame 1 or, alternatively, to the base 12 of the chair, by means of a reduction gear unit 9 with cogged wheels 9 a, 9 b. More precisely, the revolving motion of the handle 8 around the axis of the relative support shaft 13 is transmitted to the control rod 11, at least provided with a portion having a worm screw conformation, by means of the same shaft 13, integral to a first cogged wheel 9 a that meshes with a second cogged wheel 9 b, in turn integral to the control rod 11.

As will be clear to those skilled in the art, possible replacement of the worm screw 11 with a cogged pinion and the corresponding replacement of the mobile helical groove head 10 with a rack that engages the teeth of the aforesaid cogged pinion, is included in the category of the possible alternative solutions that are part of the inventive scope of the present patent application.

In a similar manner, the kinematic inversion of the worm screw 11 and relative helical groove head 10 does not substantially modify the technical solution described herein.

The operation of the illustrated mechanism is as follows.

In the absence preferably, but not necessarily, of a load acting on the backrest frame 2, the rotation, of the knob 8 in a clockwise direction performed by the user, will determine the corresponding rotation of the shaft 13 and of the cogged wheel 9 a and therefore the rotation, in an anticlockwise direction, of the wheel 9 b and of the worm screw 11.

This latter rotation of the worm screw 11, because of the meshing with the helical groove head 10, determines the roto-translation of the same mobile head 10 along the regulating rod 4 towards the sleeve 5, with the relative approach of the resting disk 6 to the mobile striker 7, and the consequential compression of the helical cylinder spring 3. It can be seen that the roto-translation of the mobile head 10 on the rod 4, aided by the preferred threaded type coupling, does not involve any rotation of the guide rod 4 with respect to sleeve 5, thanks to the square section of the hole 15 of said sleeve 5 and of the end of the rod 4, which is engaged in said hole 15.

This variable compression of the spring 3 (pre-load) determines the elastic response of the spring in the case of any further compression strain. With the increase in the compression of the spring 3, there is an increase in the force of the elastic reaction of the same spring 3 on the frame 2 and therefore this increases the stiffness of the backrest of the chair.

The application of a load on the backrest of the chair, and therefore also on the backrest frame 2, causes the rotation of the backrest frame with respect to the seating frame 1 and the aforesaid increased compression strain of the spring 3, with the relative contrast to the rotation of the frame 2 performed by said spring 3.

On the other hand, preferably in the absence of a load on the frame 2, the anticlockwise rotation of the knob 8 causes, by means of the reduction gear unit 9, the rotation of the worm screw 11 in a clockwise direction and the resulting roto-translation of the resting disk 6 along the regulating rod 4, operated by means of the helical groove head 10.

In this case, the translation of the regulating cursor 6, 10 occurs by moving away from the sleeve 5, and therefore causes the resting disk 6 to move away from the mobile striker 7, together with the relative distension of the helical cylinder spring 3.

This variation in the pre-load of the spring 3 makes less stiffen the backrest of the chair which implements the mechanism herein illustrated.

As will be clear to those skilled in the art, the use of a guide rod 4, fixed both with respect to the regulating cursor 6, 10 of the pre-load of the stiffening spring 3 for the mobile element 2 of a chair, as well as with respect to the said mobile element 2, provides a precise regulation of the preload, that is exempt of any jamming, and at the same time, allows the mechanism to be made in a compact and structurally solid manner.

The advantageous application of the gear means 10, 11 for the motion transmission—made effective because of the fact that the rod 4 is in a fixed position during the movement of the regulating cursor 6, 10 and of the sleeve 5—permits in particular a precise regulation of the pre-load of the stiffening spring 3, without jamming.

Furthermore, the use of a telescopic system 4, 5 combined with the aforesaid gear means 10, 11 makes the mechanism for regulating the preload of a stiffening spring of the present invention, particularly compact and structurally simple to construct. 

1. Mechanism for regulating the preload of at least one stiffening spring for a revolving element with respect to at least a support frame of a seating device, of the type comprising a mobile striker for one end of said at least one spring, connected to said revolving element, and a regulating cursor onto which the other end of said at least one spring rests, said regulating cursor being sliding along a rod-shaped guide element, wherein said rod-shaped guide element is constrained, at one of its ends, in a sliding manner to said mobile striker, and it is also integrally fixed to said support frame.
 2. Mechanism according to claim 1, wherein said regulating cursor comprises a mobile head and a resting element for said end of said at least one spring, said mobile head and said resting element being constrained, at least in a sliding manner, to said rod-shaped guide element.
 3. Mechanism according to claim 2, wherein said mobile head is constrained to said guide rod by means of a threaded type coupling.
 4. Mechanism according to claim 2, wherein said resting element is constrained to said guide rod such a way to be free to slide thereon.
 5. Mechanism according to claim 1, wherein said guide rod is fixed to said support frame at, or near, the end of said guide rod not engaged with said mobile striker.
 6. Mechanism according to claim 1, comprising at least one manual actuation device kinetically connected to a drive shaft, which rotates around its own axis, said manual actuation device being linked to said support frame, wherein it comprises means to transform the rotating motion of the drive shaft of said manual actuation into an at least a transversal motion of said regulating cursor along said rod-shaped guide element.
 7. Mechanism according to claim 6, wherein said drive shaft and said regulating cursor are incident with respect to one another.
 8. Mechanism according to claim 6, wherein said regulating cursor comprises a mobile head externally provided with grooves that are substantially helical, and said drive shaft comprises a worm screw, said head having grooves that are substantially helical and said worm screw also belonging to said means for transforming the rotary motion.
 9. Mechanism according to claim 6, wherein said means for transforming the rotary motion of said drive shaft comprise a cogged pinion that engages with a relative rack.
 10. Mechanism according to claim 1, wherein said mobile striker engages with said rod-shaped guide element in a telescopic manner.
 11. Mechanism according to claim 10, wherein said mobile striker comprises, or is integral, to a sleeve, inside which said rod-shaped guide element is engaged.
 12. Mechanism according to claim 11, wherein said sleeve is pivoted in a direction transversal to the axis of said rod-shaped guide element, on said mobile element.
 13. Mechanism according to claim 1, wherein said spring is a torsion cylinder helical spiral spring.
 14. Mechanism according to claim 13, wherein said cylindrical spring is mounted in a substantially co-axial manner on said rod-shaped guide element.
 15. Mechanism according to claim 1, wherein said revolving element is the backrest frame of a seating device and said support frame is the seating frame and/or the base of said seating device.
 16. Mechanism according to claim 1, wherein said seating device is an office chair. 